Question 131
Find the sum of all positive integers $n$ such that $1+2+\cdots+n$ divides
$$
15\left[(n+1)^2+(n+2)^2+\cdots+(2 n)^2\right] .
$$
$$
15\left[(n+1)^2+(n+2)^2+\cdots+(2 n)^2\right] .
$$
MathNet 2018-part-07 — 177 problems (EN–VI) Question 131
Math Olympiad Vault · MathNet 2018-part-07 — 177 problems (EN–VI)